Q49382: Example of Passing BASIC String Descriptor to MASM (Far)

Article: Q49382
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom H_MASM S_QuickASM | mspl13_basic
Last Modified: 13-AUG-1990

The two programs shown below demonstrate how a Microsoft BASIC program
passes a string descriptor to assembly language by far reference.

This information about interlanguage calling applies to QuickBASIC
versions 4.00, 4.00b, and 4.50 for MS-DOS, to Microsoft BASIC Compiler
versions 6.00 and 6.00b for MS-DOS and MS OS/2, and to Microsoft BASIC
Professional Development System (PDS) versions 7.00 and 7.10 for
MS-DOS and MS OS/2.

For more information about passing other types of parameters between
BASIC and MASM, search in the Software/Data Library for the following
word:

   BAS2MASM

Code Example
------------

The following BASIC program is BSTRF.BAS, which passes a string
descriptor (of a variable-length string) to assembly language by far
reference:

   A$ = "This is the String" + "$"    ' "$" terminates the string for
                                      ' INT call
   CALLS RString(A$)   ' CALLS passes far address
   END

The following program is ASTRF.ASM, which gets a BASIC string
descriptor, then prints the string:

; The following handy .MODEL directive is found in MASM 5.10 but not
; in earlier versions:
.MODEL MEDIUM, BASIC
.CODE
        PUBLIC RString
RString PROC
        push bp
        mov bp, sp       ; set stack frame
        push ds
        mov ds, [bp+8]   ; segment of descriptor
        mov bx, [bp+6]   ; offset of descriptor
        mov dx, [bx+2]   ; address of actual string
        mov ah, 9        ; DOS interrupt to print string
        int 21h
        pop ds
        pop bp
        ret 4
RString ENDP
        END

To demonstrate these programs from an .EXE program, compile and link
as follows:

   BC BSTRF.BAS;
   MASM ASTRF.ASM;
   LINK BSTRF ASTRF;

BSTRF.EXE produces the following output:

   This is the string

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